System and method for monitoring vibration isolators

ABSTRACT

A system may monitor a vibration isolating connection between a first part and a second part. The system may include a light source, an optical sensor mounted to receive light from the light source, and a processing unit for providing an output indicative of the deformation of the vibration isolating connection based on the output of the optical sensor.

FIELD OF THE INVENTION

The present application relates to a low cost apparatus for monitoringthe performance of vibration isolators, and a related method.

BACKGROUND OF THE INVENTION

There are many applications for vibration isolators. For example, thesuspension system of any vehicle comprises a number of vibrationisolators. In particular, a suspension system commonly comprises atleast one spring and at least one damper mounted in a suspensionstructure, which together allow significant movement of a wheel relativeto the vehicle so as to isolate the car from vibrations caused by thewheel traveling over an uneven surface.

When a vehicle is moving over a surface, the surface can haveirregularities, such as bumps or pot holes, which will cause a wheel ofthe vehicle to jolt up or down when following the surface. This suddenmovement (which is also called a mechanical shock—or shock for short)will be passed directly onto the vehicle, which can cause a lot ofdiscomfort for a person in the vehicle. Typically, a spring is used toabsorb shocks by either compressing or extending. This allows the wheelto jolt, which in turn helps the wheel remain in contact with a surface,but also to prevent shocks from being transferred to the body of thevehicle. There are many different types of springs used on vehicles—coilsprings, leaf springs, air springs, and torsion bars are some examplesof springs used in typical suspension systems.

However, when a spring absorbs a shock it dissipates the energy of theshock through oscillation. On impact it will start to oscillate andcontinue to oscillate at its resonant frequency until the energy of theshock has been fully dissipated. This oscillation will be transferred tothe body of the vehicle and will also reduce the grip a wheel exerts ona surface. In addition, this oscillation can also reduce the stabilityof the vehicle on the road. In order to control this oscillation, adamper (which is also called a shock absorber) is used. The damper'sfunction is to absorb energy from the spring.

For most road vehicles, the shocks of greatest magnitude are receivedalong a vector which can be predicted (the main shock vector). Thespring and damper are mounted along the path of the main shock vector.The largest component of this vector is a vertical component (i.e.perpendicular to the surface of a road). However, not all shocks arereceived along the main shock vector. For example, when travelling overa bump in a road, the wheel of a vehicle may be pushed by a forcecomprising a vector perpendicular to the main shock vector. This canresult in small shocks and vibrations that cause discomfort, noise, andwear of vehicle parts.

In order to absorb these additional shocks and vibrations, one or morevibration isolating connectors are used. In the context of the presentdisclosure, a vibration isolating connector is an element comprising oneor more resilient materials which is used as an interface in theconnection between two parts. Deformation of the vibration isolatingconnector allows a small amount of movement between the two parts. Thisin turns allows the vibration isolating connector to absorb small shocksand vibrations. Thus, vibration isolating connectors are particularlyuseful as part of the connection between elements of the shock absorberstructure. A vibration isolating connector can also be used for the samepurpose as part of an engine mount to minimize the amount of vibration,which is transferred from the engine of a vehicle to the chassis of thevehicle. In addition a vibration isolating connector can be used tomount the chassis of a vehicle onto the frame of a vehicle to minimizethe amount of vibration, which is transferred from the frame of avehicle to the chassis of the vehicle.

A common type of vibration isolating connector is a bushing. A bushingcan be formed comprising any suitable resilient material. For example,rubber and polyurethane are both commonly used. In addition, a bushingcan comprise a fluid. The fluid is preferably allowed to pass from onechamber to another via a channel. This type of bushing is known as ahydrobush.

Unfortunately, vibration isolating connectors degrade with use and overtime. Degradation of vibration isolating connectors may reduce theperformance of a suspension system and can potentially leave the vehicledangerous to drive. At present, vibration isolating connectors aremerely visually inspected to decide if they should be replaced. However,there are several problems with visual inspection.

For example, a skilled person, such as a mechanic must perform thevisual test. As a result, the visual test can only be performedperiodically and at a cost to the vehicle owner/user. There may be arisk that a vibration isolating connectors can fail between tests,resulting in the vehicle being used in a compromised state until thenext test—which increases the wear on the suspension system as a wholeand in turn reduces the life span of other components in the suspensionsystem.

In addition, the visual test is performed on a static vehicle. As aresult, any problems that only manifest when a vibration isolatingconnector is under load and the vehicle is moving will not be detected.Furthermore, the visual test may not pick up any internal damage to thevibration isolating connector and is likely to be overly concerned withsuperficial exterior damage. As a result, a vibration isolatingconnector with internal damage may be left in place, compromising theperformance of the suspension system. Vibration isolating connectorswill also be replaced unnecessarily because its surface looks to be inpoor condition, even if the bushing has plenty of working life left.

Another drawback may be that a suspension system can comprise a largenumber of vibration isolating connectors and other parts, some of whichmay be difficult to visually inspect. Thus, when a driver notices ahandling issue or excessive vibration within the chassis and takes thevehicle for inspection, it can be difficult to diagnose the exactproblem or determine which vibration isolating connector is the sourceof the problem. As a result, there is an increased risk of the failureof vibration isolating connectors being misdiagnosed in a test.

SUMMARY OF THE INVENTION

According to one aspect of the present disclosure, a system formonitoring a vibration isolating connection between a first part and asecond part is disclosed. The system may comprise a light source, anoptical sensor mounted to receive light from the light source, and aprocessing unit for providing an output indicative of the deformation ofthe vibration isolating connection based on the output of the opticalsensor.

For example, the optical sensor may comprise a single photon avalanchediode (SPAD), and the light source may comprise a laser diode. In someembodiments, the light source and the optical sensor may be mountedwithin a housing.

In certain embodiments, the vibration isolating connection may comprisea bushing. The light source and optical sensor nay be mounted on thefirst part. The light source may be mounted adjacent to the opticalsensor. The light source and optical sensor may be mounted together in asingle housing. Alternatively, the light source may be mounted on thefirst part, and the optical sensor may be mounted on the second part.

Additionally, the light source and the optical sensor may be mounted onthe first part, and a light guiding means or a light guide may bemounted on the second part, wherein the light guide is for directinglight from the light source to the optical sensor. For example, thelight guide may comprise a mirror. The optical sensor and the lightsource may also be mounted within the vibration isolating connection.

In some embodiments, the system may comprise a plurality of opticalsensors. The processing unit may be coupled to a first memory forstoring the output indicative of the deformation of the vibrationisolating connection. The processing unit may be coupled to a secondmemory, wherein the second memory stores information about the vibrationisolating connection.

More specifically, the processing unit may compare information in thesecond memory with the output indicative of the deformation of thevibration isolating connection. The processing unit may be adapted togenerate a warning signal when the output indicative of the deformationof the vibration isolating connection is outside of a predeterminedrange based on the information in the second memory.

In some embodiments, the second memory may be removable. The secondmemory may be adapted for communication with an external processor.

Another aspect is directed to a method for monitoring the performance ofa vibration isolating connection. The method may comprise transmittinglight from a light source along an optical path to an optical sensor,wherein the length of the path is varied by deformation of the vibrationisolating connection, and determining from the received light the avalue indicative of the length of the optical path.

The method may comprise determining the deformation of the vibrationisolating connection based on a plurality of values indicative of thelength of the optical path. The method may comprise a calibration stepfor providing a first set of readings for the vibration isolatingconnection.

Additionally, the first set of readings may comprise data regarding thecomposition of the vibration isolating connector and/or its expectedtolerances. The method may comprise comparing the value indicative ofthe length of the optical path with the first set of readings. Themethod may comprise determining if the range of motion of the vibrationisolating connector while in use is within calibrated normal valuesbased on the comparison between the value indicative of the length ofthe optical path and the first set of readings.

The method may comprise generating a warning signal if it is determinedthat the range of motion of the vibration isolating connector while inuse is within calibrated normal values lies outside calibrated normalvalues. The method may comprise recording information based on the valueindicative of the length of the optical path. For example, thetransmitting of light may be preformed at intervals of 5-100 Hz.

Another aspect is directed to a vehicle comprising the system describedabove. Another aspect is directed to a storage device adapted to cause acomputer system to execute the method described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure will now be described by way ofexample only and with reference to and as shown in the accompanyingdrawings, wherein:

FIG. 1 shows an example embodiment of a suspension system showing anumber of possible locations of vibration isolating connectors,according to the present invention.

FIGS. 2a-2b illustrate an embodiment of an SPAD based proximity sensorand an associated waveform diagram, according to the present invention.

FIG. 3 shows a block diagram of a proximity sensor, according to thepresent invention.

FIG. 4 shows an optical device mounted to measure the performance of abushing, according to the present invention.

FIGS. 5a, 5b, and 5c show images of a number of mounting positions whichcan be monitored using the system, according to the present invention.

FIGS. 6a and 6b shows an alternative mounting arrangement of opticaldevices to measure the performance of a bushing, according to thepresent invention.

FIG. 6c shows another embodiment of optical devices, according to thepresent invention.

FIG. 7 shows a flowchart for the method of monitoring the performance ofa vibration isolating connector, according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exemplary suspension system 100. As is apparent fromFIG. 1, a suspension system can comprise a large number of vibrationisolating connectors. For example, the suspension system 100 maycomprises shock and strut bushings 101, coil spring isolators 102, swaybar bushings 103, engine mounts/inserts 104, sway bar end link bushings105, control/trailing arm bushings 106, leaf spring bushings 107, bumpstops 108, shock and steering stabilizers 109, chassis mounts 110, rackand pinion bushings 111, radius/strut arm bushings 112, tie rid boots113, transmission mounts 114, control arm bushings 115, and ball jointboots 116. Of course, other arrangements of vibration isolatingconnectors are possible.

FIG. 1 shows a typical chassis to which bodywork would be attached.Alternatively, vehicles may employ subframes in the region of each axle.The subframes may be either directly bolted to the body, or viaadditional bushings to allow some movement between subframe and chassis.Subframe-body bushings may be well hidden and difficult to observe bytypical methods, so are particularly suited to being monitored using thepresent embodiments. In the present disclosure, an optical sensor and alight source are used to monitor the performance of a vibrationisolating connector.

An SPAD is particularly well suited for this application. An SPAD isbased on a p-n junction device biased beyond its breakdown region. Thehigh reverse bias voltage generates a sufficient magnitude of electricfield such that a single charge carrier introduced into the depletionlayer of the device can cause a self-sustaining avalanche via impactionization. The avalanche is quenched, either actively or passively toallow the device to be “reset” to detect further photons. The initiatingcharge carrier can be photo-electrically generated by way of a singleincident photon striking the high field region. It is this feature whichgives rise to the name “single photon avalanche diode.” This singlephoton detection mode of operation is often referred to as “GeigerMode.”

The idea that an SPAD can be used as in a ranging application is borneout by the application of a phase shift extraction method for rangedetermination, although alternative methods exist for rangedetermination using SPADs based on direct time of flight measurement.The term ranging in this application is intended to cover all rangingdevices and methods including by not limited to ranging devices,proximity devices accelerometers, etc. Ranging can occur in a number ofapplications, including proximity detection which is relatively easy toimplement and inexpensive. Laser ranging is more complex and costly thana proximity detector. Three-dimensional imaging is a high-endapplication that could be used to recognize gestures and facialexpressions.

A proximity sensor is the most basic of the ranging applications. At itssimplest, the sensor is capable of indicating the presence or absence ofa user or object. Additional, computation and illuminator complexity canprovide enhanced data such as the range to an object. A typical range isof the order 0.01 m to 0.5 m. In a simple proximity sensor, theillumination source could be a modulated LED, at a wavelength of about850 nm.

The next application group is that of laser ranging, where theillumination source is a modulated diode laser. Performance can rangefrom <1 cm to 20 m range (and higher for top end systems) withmillimeter accuracy. Requirements on optics are enhanced, withhemispherical lenses and narrow band pass filters being required. Anear-field return may results in the introduction of parallax error,i.e. movement of the returned laser spot over the sensor pixel arraydependent on distance to object. To overcome these problems the rangerincludes calibration functions to enable the subtraction of theelectronic and optical delay through the host system. Laser ranging isparticularly advantageous when monitoring a vibration isolatingconnector because they do not deform significantly. As a result, unlessa sensor is able to detect movement within the range of 2 mm-25 mm, itnot be able to pick up the deformation of a vibration isolatingconnector accurately.

The third application group is that of 3D cameras. In this application apixel array is used in order to avoid mechanical scanning of the array.Systems can be based on a number of different architectures. Both timeof flight (TOF) and modulated illuminator based architectures are used,however, the latter is more robust to ambient light and thus fits bestwith established photodiode construction. Additional features, such asface and gesture recognition, are applications of this type of rangingdevice.

Most optical ranging implementations use either stereoscopic, structuredlight, direct TOF or phase extraction methods in order to ascertain therange to a target. Stereoscopic solutions use two typical cameras, andcan have a heavy computation overhead in order to extract range. Thestructured light scheme uses diffractive optics and the range iscomputed using a typical camera based on how a known projected shape ormatrix of spots is deformed as it strikes the target. The direct TOFmethod uses a narrow pulsed laser, with a time-digital converter (TDC)measuring the difference in time between transmission and first photonreception. Commonly, a “reverse mode” is employed, where the TDCmeasures the back-portion of time, i.e. the time from first photonreception to next pulse transmission. This scheme minimizes systemactivity to only the occasions where a photon is detected, and istherefore well matched to tightly controlled, low photon flux levels andmedical applications such as fluorescent lifetime microscopy (FLIM).

The phase extraction method is probably the most commonly used method asit is well suited to systems which implement computation of thegeneralized range equation using existing photodiode technology. It isalso robust to background ambient light conditions, and may be adaptedto allow for varying illuminator modulation wave-shapes (i.e. sinusoidalor square). This scheme is favored for SPADs in proximity detectionapplications. The present embodiments take advantage of the fact thatthe phase extraction method system incorporates an inherent ambientlight level detection function, which can be used in conjunction with anSPAD for many applications.

It is important to understand the range equation derivation as itindicates the ease of applicability of SPADs to phase extractionproximity detection and ranging solutions. It also aids in theunderstanding of inherent features such as ambient light metering andmeasuring a depth of interest for a specific purpose.

Distance is determined from the speed of light and TOF, as follows:s=ct;where s is distance, c the speed of light, and t is time. For a rangingsystem however, the distance is doubled due to the fact there are sendand receive paths. As such, the distance measured in a ranging system sis given by:s=½ct.

The time shift component (=“t”) due to the photon TOF is dependent onthe modulation frequency and phase shift magnitude of the waveform. t=%shift of the returned waveform xt_(mod) _(_) _(period) and if t_(mod)_(_) _(period)/f_(mod):

$\left. \Rightarrow t \right. = {\left. {\frac{\phi}{2\;\pi} \circ \frac{1}{f}}\Rightarrow t \right. = {\frac{\phi}{2\;{\pi \circ f}}.}}$The units are in radians. Then, by substituting the above equation backinto the starting equation: the “range equation” is expressed as:

$\left. \Rightarrow s \right. = {\frac{c \circ \phi}{4\;{\pi \circ f}}.}$The critical component in this equation is φ, which is the unknowncomponent of the % shift of the returned waveform. The following sectiondiscusses how this can be determined.

Since the values of c, f and π are all constants; the range resultsimply scales with (I), (the % shift of the received light waveform inrelation to that which was transmitted). FIG. 1 demonstrates how φ maybe determined for a system employing a square wave modulatedilluminator. The transmitted and received waveforms are shifted from oneanother by φ. By measuring the photons that arrive in “a” and “b,” inbins 1 and 2 respectively, the value of φ can be determined as follows:

$\frac{\phi}{2\pi} = {\frac{b_{count}}{\left( {a + b} \right)_{count}}.}$

In this type of system, there is a range limit set by the illuminatormodulation frequency, which is known as the unambiguous range. Photonsreceived from targets that are further away than this range canintroduce an aliasing error by erroneously appearing in a legitimate binfor a subsequent measurement. Since determination of range is enabled bythe modulation process, it is desirable to maximize the number of edgesof the modulation waveform in order to accumulate data for averagingpurposes as fast as possible. However, a high modulation frequency maylower the unambiguous range and introduces more technical complexity inthe illuminator driver circuitry. Therefore, two or more differentmodulation frequencies may be interleaved or used intermittently, so asto reduce or negate the impact of aliased photons via appropriate dataprocessing.

FIG. 2A illustrates a possible implementation of an SPAD based proximitysensor with an associated waveform diagram. FIG. 2A shows an SPAD 200connected to a multiplexer 202. The output from the multiplexer passesthrough counters 1 and 2 (204). The SPAD device, shown generally at 200,is of a typical type, including a photo diode 210, a p-type MOSFET 212,and a NOT gate 214.

The timing waveforms are shown in such a way so as to represent therelative photon arrival magnitudes. It can be seen that an extra phasehas been added to enable computation of the background ambient lightlevel offset “c,” although this can be significantly reduced by the useof a narrow optical band-pass filter matched to the illuminatorwavelength if necessary. The element “c” is then accommodated in thecomputation of received light phase shift φ. The computed results for a,b, c are determined and written into either a temporary memory store oran I2C register. The computation of the phase shift φ, is calculated asfollows:

$\phi = {\frac{a_{count} - c}{\left( {a + b} \right)_{count} - {2c}}.}$

The predetermined selection of modulation frequency is performed bydedicated logic or host system, which selects a suitable frequency orfrequencies for the application of the range sensor. The range sensor ofFIG. 2A is dependent on the amount of light that can be transmitted onto the scene, system power consumption and the target reflectivity.

Since the system shown in FIG. 2A needs to compute the background lightcondition in order to ascertain the offset of the returned light pulsefrom the target, ambient light metering is included. A simplified timingscheme is employed if only the ambient light level data is required,since the target illumination cycle is not necessary. If a narrow bandIR filter is employed in the optical path, the value of c will representonly the content of the filter pass band. This can then be extrapolatedto an approximation of the general ambient light conditions.

Referring to FIG. 3, a block diagram of a proximity sensor is shown. Theproximity sensor 300 includes SPAD function and the quenching thereof inblock 302. The quenching can be passive as shown or of any othersuitable type. The bias voltage for the SPAD may be provided by a chargepump or any other suitable device 304. The sensor module also includesan LED or other illumination source and an associated driver 306 toensure that the required modulation is applied to the illuminationsource.

The sensor may include a distance computation logic module to determinerange. Alternatively this can be located in a host device in which therange sensor is used. The sensor also includes multiplexers and counters308 and a storage means or a memory 310, such as an I2C module. Thesensor may also include a Phase Locked Loop (PLL) for clocking andsubsequent timed signal generation purposes.

The power consumption of SPADs and their readout circuits is dependenton the incident photon arrival rate. The average power consumption of aranging system could be reduced by using power saving modes such aspulsed on/off operation, at a rate of ˜10 Hz, for example, at theexpense of target motion distortion.

The sensor may be implemented on a 1 mm² die size, and the I2C modulecould also be implemented on an appropriate die. The sensor may includean optical package, an integral IR band pass filter (either coating orinherent in the optical elements) and an optimal field of view of about30°. As the sensor is not intended to “create an image” but is insteadused to ensure that as many photons as possible are detected the opticscould be made from injection molded hemispherical elements. Theilluminator source should ideally be of a non-visible wavelength, forexample, in the near infra red (NIR) band, such as 850 nm.

The proximity sensor has been described with reference to simple lowcost system, although it will be appreciated for certain applications,the laser ranging and 3D camera technologies discussed above, could beused. As previously indicated, the proximity sensor of the presentembodiments is versatile and can be used in a vast array of differentapplications.

The measurement of photons that arrive in “a” and “b,” in bins 1 and 2respectively, and hence the value of φ is determined by a dataextraction algorithm, which is performed on the output of the opticalmodule 12. Thus, the detection of an object in proximity to the opticalkey generates a keystroke signal in a similar manner to a pressing a keyof a typical keyboard.

FIG. 4 shows a shock and strut bushing in more detail. The bracket 403is connected to a McPherson strut and is for mounting the strut to thearm 401. The arm 401 has an opening for receiving a cylindrical bushing404. The bracket 403 is bolted or pinned to the arm 401 such that thestem 405 of the bolt or pin runs through the bushing 404. As a result,sudden shocks are transferred in a chain from the arm 401, to thebushing 404, to the stem 405, and to the bracket 403. The bushing 404acts as vibration isolating connector. The bushing 404 deforms undershock allowing a small amount of movement in the range of 1 mm to 15 mmin the position of the stem 405 relative to the arm 401. As a result,the bracket's 403 position relative to the arm 401 will also changeunder shock.

An optical sensor 407 is mounted on the arm 401 and a light source 406is mounted on the bracket 403 to form a light source/optical sensorpairing. In advantageous embodiments, the optical sensor 407 is an SPADas discussed above, and/or the light source 406 is a laser diode.

Other arrangements of the light source/optical sensor pairing arepossible. For example, the positions of the optical sensor 407 and thelight source 406 can be swapped, or the light source/optical sensorpairing can be positioned at a different point around the periphery ofthe bracket 405. So long as light source/optical sensor pairing is setup such that: the optical sensor 407 detects light from the opticalsource 406; and the optical sensor 407 is located on a part which movesrelative to the optical source 406 due to deformation of the bushing404, movement due to deformation of the bushing 404 can be detected. Asthe arm 401 moves up and down, the bracket 403 will also rotate relativeto the arm 401. So long as the magnitude of the angle of rotation issmaller than the angle α of the light beam having its apex at the lightsource 406, this will not present a problem.

It will also be apparent that other vibration isolating connectors canbe monitored in the same way, i.e. so long as a light source/opticalsensor pairing is set up such that: the optical sensor detects lightfrom the optical source; and the optical sensor is located on a firstpart which moves relative to a second part to which the optical sourceis mounted. The relative movement is due to deformation of the vibrationisolating connector—the performance of the vibration isolating connectorcan be determined.

Mirrors and lenses can also be employed. For example, a washer havingouter cylindrical surface which is mirrored can be located on the stem405. It can be located between the bracket 403 and the nut or fixingwhich holds the bolt/pin in place (not shown) or alternatively betweenthe arm 401 and the bracket 403. In this case, the light source andoptical sensor can be located beside each other on the arm 401. Lightfrom the light source is directed towards the washer so that it isreflected back to the light sensor. As the mirrored washer will moverelative to the arm due to deformation of the bushing 404, this movementcan be detected. In this case, rotation of the bracket 403 relative tothe arm 401 will not cause a problem because rotation of the mirroredwasher will not alter its reflexive characteristics due to itscylindrical shape, i.e. it will reflect in the same way whether itrotates or not.

For all the different arrangements of the present embodiments, mirrorsand lenses can be further employed to change the optical path of thelight from the light source to the optical sensor, so that the opticalpath need not follow a straight line. Preferably, the lightsource/optical sensor pairing are housed in a single package, whichshines light towards the part to be monitored and detects lightreflected back from a reflecting surface on the part. In addition, it isalso possible to mount a light source/optical sensor pairing within avibration isolating connector itself.

For all the different arrangements of the present disclosure, two ormore light source/optical sensor pairings, each pairing having its ownlocation separate from the other pairings, can be used. This allows thesystem to generate 2-dimensional positional data using trigonometry forthe movement due to deformation of the bushing 404. The use of aplurality of light source/optical sensor pairings can also improve theaccuracy of the system.

For all the different arrangements of the present disclosure, the wheelarch and the vehicle chassis of a vehicle should protect a lightsource/optical sensor pairing from getting covered by ambient dirt ormud from the surface upon which the vehicle is travelling. FIGS. 5a, 5b,and 5c show images of a number of mounting position which can bemonitored using the present system. These images were taken from a car;FIG. 5a illustrates an example site for monitoring the deformation of afront control arm inner bush. FIG. 5b illustrates an example site formonitoring the deformation of a front wishbone inner bush, and FIG. 5cillustrates an example site for monitoring the deformation of a frontanti-roll bar mount.

This car is over 5 years old and was used through Scottish wintersincluding trips to ski areas. Surprisingly, there is a lack of dirtbuildup in the areas proposed for measurement. Thus, a lightsource/optical sensor pairing can be mounted without protection from theingress of dirt, thereby lowering the cost of the system.

However, the light source/optical sensor pairing can alternatively becontained within a housing if required to ensure that no dirt caningress and interfere with the transmission and reception of light. Inorder to accommodate the movement of a vibration isolating connector,the housing can be formed of a resilient material. Alternatively, thehousing can comprise two slidably engaged members, which can moverelative to each other in use. Alternatively, the housings can comprisea flexible bellows structure.

FIGS. 6a and 6b show an alternative arrangement for mounting lightsource/optical sensor pairings. In this arrangement, packages 607 a and607 b are provided on a subframe 603 (which is not shown in FIG. 6b forclarity). Each package comprises a light source and an optical sensor.The light source shines light towards arm 601, and the optical sensordetects light reflected back from a reflecting surface on the arm 601.The reflecting surface must be suitably shaped such that it remains atan approximately constant distance from the measuring device undercontrolled suspension and steering movement. Measurements are made ofthe uncontrolled movement allowed by the bushing material to absorbshocks. Package 607 a is positioned for measuring deformation of abushing 604 along a first axis substantially at right angles to stem605. Package 607 b is positioned for measuring deformation of thebushing 604 along a second axis substantially parallel to stem 605. Inthis way, a two dimensional picture of the deformation of the bushing604 can be obtained.

FIG. 6c shows a further arrangement for mounting light source/opticalsensor pairings. In this arrangement, packages 607 a and 607 c areprovided on a subframe 603. Again, each package comprises a light sourceand an optical sensor. The light source shines light towards arm 601,and the optical sensor detects light reflected back from the arm 601.Package 607 a is positioned for measuring deformation of a bushing 604along a first axis substantially at right angles to stem 605. Package607 b is positioned for measuring deformation of the bushing 604 along athird axis which is substantially at right angles to both the first axisand the stem 605.

It should be noted that any number of light source/optical sensorpairings could be used. In addition, any combination of the aboveembodiments could also be used. In particular, three packages, such as607 a, 607 b, and 607 c, could be provided so that: a first package 607a is positioned for measuring deformation of a bushing along a firstaxis substantially at right angles to a stem; a second package 607 b ispositioned for measuring deformation of the bushing along a second axissubstantially parallel to the stem; and a third package 607 c ispositioned for measuring deformation of the bushing along a third axis,which is substantially at right angles to both the first axis and thesecond axis, thereby allowing the deformation of the bushing to bemonitored in three dimensions.

As noted above in relation to FIG. 1, subframe-body bushings may be wellhidden and difficult to observe by traditional means, so areparticularly suited to being monitored using the present system. In thiscase, at least one light source/optical sensor pairing is preferablymounted as a package on either the subframe or the vehicle body tomonitor the relative distance between the subframe and the vehicle body,which is due to deformation of the subframe-body bushing(s). Of course,more than one light source/optical sensor pairing can be used in amanner similar to that described in relation to FIGS. 6a, 6b , and 6 c.

FIG. 7 shows a flow chart for the method of monitoring the performanceof a vibration isolating connector. In step 701, the system iscalibrated. Data indicative of the readings of one or more lightsource/optical sensor pairings for a new vibration isolating connectoris provided to a memory located in the vehicle to which the vibrationisolating connector is mounted. In addition, data regarding thecomposition of the vibration isolating connector and/or its expectedtolerances can also be provided to the memory.

In step 702, when the vehicle is in use, the readings of the one or morelight source/optical sensor pairings are recorded continually or atregular predetermined intervals. The readings can then be provided to acontroller. Preferably, the rate of measurement is in the range of 5-100Hz. An optimum sample rate may also be calibrated by sampling at thehighest frequency possible to record the shortest duration ofdisplacement during test conditions.

In step 703, the readings provided in step 702 are compared continuallyor at regular predetermined intervals against the stored calibrationdata to determine if the range of motion of the vibration isolatingconnector while the vehicle is in use is within calibrated normalvalues. Based on this determination, a decision is made as to whetherany of the readings provided in step 702 exceeds a satisfactory workingrange.

If an out-of-range reading is received, then a warning signal isgenerated in step 704. This signal can be provided to a display or lighton the dashboard of the vehicle so that it is raised on the dashboard.Alternatively, the signal can be used to generate a warning sound in thecabin of the car. A record of out-of-range data can be stored forsubsequent upload and diagnosis at an appropriate vehicle servicecenter. Steps 702, 703, and 704 can be executed by one or moreprocessors coupled to memory.

It will be appreciated that the aspects of the suspension system andmethod discussed herein can be used in a large number of applicationswhere vibration isolating connectors are used to dampen vibrations. Forexample, the present disclosure may have applied to engine suspensionsystems which employ bushings as part of the mounting between an engineand a vehicle chassis.

That which is claimed is:
 1. A method for monitoring a vibrationisolating connection, the method comprising: transmitting light from alight source along an optical path to be received by a single photonavalanche diode (SPAD) optical sensor, a length of the optical pathbeing varied by deformation of the vibration isolating connection;operating the light source to interleave a plurality of differentmodulation frequencies; and determining from received light a valueindicative of the length of the optical path.
 2. The method of claim 1further comprising determining the deformation of the vibrationisolating connection based upon a plurality of values indicative of thelength of the optical path.
 3. The method of claim 1 further comprisingperforming a calibration to provide a first set of readings for thevibration isolating connection.
 4. The method of claim 3 wherein thefirst set of readings comprises data for a composition of the vibrationisolating connection and tolerances of the vibration isolatingconnection.
 5. The method of claim 3 further comprising comparing thevalue indicative of the length of the optical path with the first set ofreadings.
 6. The method of claim 5 further comprising determining if arange of motion of the vibration isolating connection while in use iswithin a range based upon the comparing between the value indicative ofthe length of the optical path and the first set of readings.
 7. Themethod of claim 6 further comprising generating a warning signal if therange of motion of the vibration isolating connection while in useexceeds a threshold.
 8. The method of claim 1 further comprisingrecording information based upon the value indicative of the length ofthe optical path.
 9. The method of claim 1 wherein the vibrationisolating connection comprises a bushing.
 10. The method of claim 1wherein transmitting of the light is performed at intervals of 5-100 Hz.11. A method for monitoring a bushing, the method comprising:transmitting light from a laser source along an optical path to bereceived by a single photon avalanche diode (SPAD) optical sensor, alength of the optical path being varied by deformation of the bushing;operating the laser source to interleave a plurality of differentmodulation frequencies; and determining from received light a valueindicative of the length of the optical path.
 12. The method of claim 11further comprising determining the deformation of the bushing based upona plurality of values indicative of the length of the optical path. 13.The method of claim 11 further comprising performing a calibration toprovide a first set of readings for the bushing.
 14. The method of claim13 wherein the first set of readings comprises data for a composition ofthe bushing and tolerances of the bushing.
 15. The method of claim 13further comprising comparing the value indicative of the length of theoptical path with the first set of readings.
 16. The method of claim 15further comprising determining if a range of motion of the bushing whilein use is within a range based upon the comparing between the valueindicative of the length of the optical path and the first set ofreadings.
 17. The method of claim 16 further comprising generating awarning signal if the range of motion of the bushing while in useexceeds a threshold.
 18. The method of claim 11 further comprisingrecording information based upon the value indicative of the length ofthe optical path.
 19. The method of claim 11 wherein transmitting of thelight is performed at intervals of 5-100 Hz.
 20. A non-transitorycomputer readable medium for causing a computer system to execute amethod comprising: transmitting light from a light source along anoptical path to be received by a single photon avalanche diode (SPAD)optical sensor, a length of the optical path being varied by deformationof a vibration isolating connection; operating the light source tointerleave a plurality of different modulation frequencies; anddetermining from received light a value indicative of the length of theoptical path.
 21. The non-transitory computer readable medium of claim20 wherein the computer system executes the method further comprisingdetermining the deformation of the vibration isolating connection basedupon a plurality of values indicative of the length of the optical path.22. The non-transitory computer readable medium of claim 20 wherein thecomputer system executes the method further comprising performing acalibration to provide a first set of readings for the vibrationisolating connection.
 23. The non-transitory computer readable medium ofclaim 22 wherein the first set of readings comprises data for acomposition of the vibration isolating connection and tolerances of thevibration isolating connection.
 24. The non-transitory computer readablemedium of claim 20 wherein the computer system executes the methodfurther comprising transmitting of the light at intervals of 5-100 Hz.